The present invention relates to an orbital optical imaging system, and in particular to a method and apparatus that controls a scanning mirror, which provides a reflected image to a multiband (i.e., multiple spectral bands) focal plane array.
Satellites are widely used as platforms for various atmospheric monitoring instruments. For example, the United States National Oceanic and Atmospheric Administration (NOAA) operates a number of satellites that are used for weather monitoring. Some of these satellites operate in a geosynchronous orbit and regularly scan over predetermined areas of the Earth in order to monitor current weather conditions and provide data to weather forecasting systems.
The optical imaging instrument (an "imager") on these satellites includes several optical spectral bands that are used to monitor the weather. For example, there may be a visible imaging band and several infrared imaging bands. These various spectral bands together provide information regarding cloud cover and various forms of precipitation (e.g., rain, snow, sleet or hail) over the area of the Earth being monitored.
Technology improvements have recently made available large arrays comprising hundreds of optical detectors, that were not available when the current weather satellites were designed and manufactured. It is now possible to combine side-by-side, on a common surface, several long, line arrays of optical detectors, with each line array having detectors responsive to a particular spectral band. This assembly is often referred to as a multiband array of detectors. If this assembly of detectors is then installed on the focal plane of an optical instrument, the assembly is called a multiband focal plane array (MBFPA). For example, the MBFPA of a weather imager may include a visible band (e.g., 0.5-0.7 micrometers) and a number of infrared bands (e.g., eight bands over the range 1.8-13 micrometers).
An engineering obstacle to the use of a MBFPA in an advanced imager is the geometric distortion due to an optical effect known as "image rotation" that results if the MBFPA is used in combination with a conventional single-mirror scanning system. The purpose of the scanning system is to project a selected portion of the Earth (the "scene") onto the MBFPA. In a conventional, single-mirror scanning system the mirror is first positioned relative to a first axis (e.g., to select the elevation of the scan), and then scanned about a second axis (e.g., azimuth) while holding the mirror position constant with respect to the first axis. The first and second axes are orthogonal. This scan process allows a user to select the Earth scene (usually in an automated sequence of scans) to be viewed by the imaging instrument. The scanning process can be repeated in a progressive sequence to obtain a multi-spectral image of the full Earth disk.
However, the use of a conventional, single-mirror scan system in combination with the MBFPA causes two undesirable errors. The first error is a pixel-to-pixel registration error within a frame, which is a geometrical distortion in which the pixels within an image frame are misaligned with respect to each other, such that they are not "observed" at their true positions in the Earth scene. The second error is a band-to-band coregistration error. The coregistration error is a geometrical effect in which the detectors in different spectral line arrays of the MBFPA do not traverse the same area of the Earth scene during the scan. Both of these errors have an adverse effect on the quality of weather imagery. Therefore, a technique is needed to image an Earth scene onto the MBFPA without introducing the registration and coregistration errors in order to utilize the new MBFPA technology effectively. Several techniques are currently under investigation.
One technique is to use an assemblage of multiple beamsplitters to split the incoming light flux onto a series of separate, single-band focal plane arrays of detectors. However, this technique does not enable the use of the MBFPA since the separate focal planes have detectors of only a single spectral imaging band. In addition, this technique does not correct for the pixel-to-pixel registration error. This design approach is also relatively costly and complex due to the precision required in the physical alignment and calibration of the assemblage of beamsplitters.
A second approach under consideration is to use two cascaded scanning mirrors. The first mirror would scan about a first axis (e.g., scan in elevation), while the second mirror would scan about a second axis (e.g., scan in azimuth). This approach would correct both registration and coregistration errors and, indeed, enable the use of the MBFPA technology. However, since the mirror is a heavy component of the imager, adding an additional mirror is undesirable because it adds to the mass and bulk of the satellite. Significantly, it adds to the launch mass of the satellite.
Therefore, there is a need for an imaging system comprising a MBFPA that is free from the undesirable geometrical errors of image registration and coregistration.